1. Field of the Invention
The present invention relates, in general, to a method for preparing electrodes for Ni/metal hydride (hereinafter referred to as xe2x80x9cMHxe2x80x9d) secondary cells and, more particularly, to the use of copper in preparing electrodes for Ni/MH secondary cells, thereby improving the performance of the electrodes.
2. Description of the Prior Art
As a rule, improvement of the negative electrodes for Ni/MH secondary cells is achieved by amelioration of active materials themselves, which consist of hydrogen storage alloys, current collectors and binders, and/or by additives. Aiming to maximize the properties the active materials themselves have, the amelioration comprises the change in alloy composition (alloy design) and the modification of alloy surface through, for example, electroless plating. In connection with the additive, the negative electrode is usually modified by taking advantage of copper (Cu) and nickel (Ni) for current collectors, and polytetrafluoroethylene (PTFE) and polyvinyl alcohol (PVA) for binders.
It is reported that the electroless-plating of Ni or Cu on the surface of electrodes can improve the low temperature dischargeability and current density dependence thereof (T. Sakai, J. Less-Common Metals, 172-174 (1991) 1175). The electroless-plating processes suggested by T. Sakai et al., however, suffer from disadvantages of being complicated and producing pollution of the environment owing to their toxic by-products.
A method of improving the general functions of electrodes by changing the properties that active materials themselves have, rather than by the additional processes, such as electroless plating, is disclosed (H. Sawa, Mat. Trans. JIM, 31 (1990) 487). According to the disclosure, the content of Ni in a hydrogen storage alloy is increased from the start of the alloy""s design. However, the discharge capacity of the electrode is found to decrease with the increasing of the Ni content.
Lee observed that the surfaces of electroless-plated alloy powders are coated with Cu or Ni particles less than 10 xcexcm in size (thesis for Ph. D. in KAIST, Taejon Korea (1995)). This electroless-plating can prevent electrodes from being in direct contact with electrolytes, thereby extending the life span of the electrodes. In addition, it is also reported that, when forming the electrodes with the alloy powders, the electroless-plating improves the electrodes in moldability and electroconductivity. However, the functional improvement of the electrodes is difficult to control because the size of the plated particles, a main factor to determine the functions, is changed depending on the conditions of the plating processes. Another significant disadvantage of the electroless plating technique is that it is difficult to prolong the plating effect for cycles of charge/discharge because the lattice expansion of the alloy, which occurs upon charging and discharging, causes the electrodes to powder, leading to the breakage of the plating layer.
Hydrogen storage alloys are the metals or alloys which are able to absorb or discharge hydrogen reversibly at certain temperatures under certain pressures. In order for the hydrogen storage alloys to be applied in practice, they are required to have reversibly available, large hydrogen storage capacities as well as show rapid hydrogenation in electrolytes.
Hydrogen storage alloys for Ni/MH secondary cells, developed thus far, can be divided largely into two types: AB5 type and AB2. type, wherein A is an element having a high affinity for hydrogen, i.e., an alkaline earth element, such as La, Ce, Ti, Zr, etc., and B is a transition metal or transition metals selected from Ni, Mn, Co, Fe, Al, etc. Each type suffers from its own disadvantage. For example, the AB5 type, of which Laxe2x80x94Ni and Mnxe2x80x94Ni are representative, is low in energy storage density while the AB2 type, exemplified by Zrxe2x80x94Ni and Tixe2x80x94Ni, is poor in its general functions.
In recent, research on the development of hydrogen storage alloys has been directed to the AB2 type on account of its security for high capacity.
Generally, the surface properties of hydrogen storage alloys are determined by alloy design, the coating and etching of alloy surfaces and/or additives. It is very difficult to design alloy compositions as being excellent in thermodynamic properties, that is, as having large hydrogen storage capacity as well as superior surface properties. The coating and etching of alloy surfaces is problematic in that the solution used needs additional processes for its treatment, which are usually carried out in harmful atmospheres. In contrast, the surface property improvement by additives is easy to apply for paste-type electrodes because useful additives can be simply mixed upon the formation of the electrodes without additional processes.
As an additive for improving the surface properties of the alloy, Cu powder has been recommended. For example, in Korean Pat. Appl""n No. 97-1526, it is disclosed that a great reduction in reaction resistance can be achieved on the surface of Cu powder-mixed electrodes. Also, the patent reveals that the reduction is attributed to the fact that the mixed Cu powder undergoes the melting and deposition during cycles of charging and discharging so that it is uniformly dispersed over the surface of the electrode to increase the effective reaction surface of the electrode. In addition, it is reported that a Cu coating of AB5 type hydrogen electrode alloy improves inner cell pressure properties (T. Sakai, J. Less-Common Metals, 172-174 (1991) 1194).
With the background in mind, the present inventors made a conception of the influence of Cu powder on electrode surface properties and inner cell pressure properties and found that electrodes with a certain fraction of Cu powder allow the secondary cells to be greatly improved in various properties, including inner cell pressure, high rate dischargeability, energy density per volume, etc., without changing the characteristic properties of the active materials consisting of hydrogen storage alloys, binders, and current collectors.
Therefore, it is an object of the present invention to provide a method for preparing electrodes for Ni/MH secondary cells which are superior in inner cell pressure, high rate dischargeability, and energy density per volume.
It is another object of the present invention to provide a method for preparing electrodes for Ni/MH secondary cells, which is simple and does not produce pollution of the environment.
Based on the present invention, the above objects could be accomplished by a provision of a method for preparing a negative electrode for Ni/metal hydride secondary cells, in which a composition comprises a hydrogen storage alloy, a binder and a current collector comprising copper.
In an embodiment of the present invention, the hydrogen storage alloy is a powder with a size of xe2x88x92400 mesh (38 xcexcm or smaller), the binder comprises a mixture of from 1:1 to 2:1 polytetrafluoroethylene: acrylic polymer in water (preferably brand name 503H of O-Gong Bond Co., Korea, a mixture of 32 wt % acrylic polymer in water) at an amount of from 2 to 4 wt %, and preferably 3.6 wt % based on the weight of hydrogen storage alloy, and hydroxypropyl methyl cellulose at an amount of from 0.3 to 0.8 wt % and preferably 0.5 wt % based on the weight of the hydrogen storage alloy, the hydroxypropyl methyl cellulose serving as a thickener, and a current collector is used at an amount of from 1 to 2.5 wt %, and preferably 1.5 wt % based on the weight of the hydrogen storage alloy.